Batteries which are used in hybrid and electric vehicles and which are referred to as traction batteries since they are used to feed electric drives are known from the prior art.
The basic circuit diagram of an electric traction drive system 10 known from the prior art, such as is used, for example, in electric and hybrid vehicles or else in stationary applications such as in the case of the rotary blade adjustment of wind power plants, is illustrated in FIG. 1. A battery (traction battery) 11 is connected to a direct voltage intermediate circuit which is buffered by a capacitor 60. Connected to the direct voltage intermediate circuit is also a pulse-controlled inverter 50 which makes available sinusoidal voltages with phases which are offset with respect to one another via, in each case, two switchable semiconductor valves 51 and two diodes 52 at three outputs, for operating an electric motor 70. In order to simplify the illustration, just one semiconductor valve and one diode are provided with reference symbols in the drawing. The capacitance of the capacitor 60 is selected to be sufficiently large to stabilize the voltage in the direct voltage intermediate circuit for a time period in which one of the switchable semiconductor valves 51 is connected through. In a practical application such as an electric vehicle, this requires a high capacitance in the range of mF.
Three-phase motors are generally used as the electric motors (electric machines) 70 in such traction drives 10. They are usually asynchronous motors, permanently regulated synchronous motors or externally excited synchronous motors. In order to feed the electric machine 70, pulse-controlled inverters 50 are generally used, said pulse-controlled inverters 50 being generally implemented in the traction region in electric and hybrid vehicles with semiconductor switches 51 which are embodied as bipolar transistors with insulated gate electrodes (IGBT).
The battery 11 which is illustrated in FIG. 1 comprises a battery module line 12 in which a multiplicity of battery cells 21 are connected in series and optionally additionally in parallel in order to obtain a high output voltage and battery capacitance which are desired for a respective application, wherein in order to simplify the illustration just one battery cell has been provided with a reference symbol. A charging and isolating device 30 is connected between the positive pole of the battery cells 21 and a positive battery terminal 22. An isolating device 40 can optionally be additionally connected between the negative pole of the battery cells and an negative battery terminal 23.
The isolating and charging device 30 and the isolating device 40 each comprise a contactor 31 or 41, which are provided to disconnect the battery cells 21 from the battery terminals 22, 23 in order to switch the battery terminals 22, 23 to a voltage-free state. On the basis of the high direct voltage of the series-connected battery cells 21, a considerable risk potential is otherwise posed to maintenance personnel or other persons. In addition, a charging contactor 32 with a charging resistor 33 which is connected in series with the charging contactor 32 is provided in the charging and isolating device 30. The charging resistor 33 limits a charging current for the capacitor 60 when the battery 11 is connected to the direct voltage intermediate circuit. For this purpose, the contactor 31 is initially left open and closed only for the charging connector 32. If the voltage appears at the positive battery terminal 22, the voltage of the battery cells 21, the contactor 31 can be closed and, if appropriate, the charging contactor 32 can be opened. The contactors 31, 41 and the charging contactor 32 do not increase the costs for a battery 11 unacceptably since high demands are made of their reliability and of the currents which are to be conducted by them.
If a technical problem occurs in the traction battery of such a traction drive, which problem can lead either directly to the failure of a battery cell or, during further operation of the battery, to a safety-relevant unsafe state of the battery, the battery is changed into a safe state by the battery management system.
This state is brought about according to the prior art in the lithium-ion battery systems by the battery being disconnected from the direct voltage intermediate circuit by opening the contactors of the charging and isolating device.
The guide system of the traction drive has to then cope with the situation that the battery is no longer available as an energy store. Depending on the operating state of the electric motor, the guide system essentially just then ensures in such a situation that the pulse-controlled inverter or inverter is not destroyed. Destruction of the inverter can be brought about, for example, by an unacceptably strong rise in the voltage of the direct voltage intermediate circuit. The guide system can consequently no longer allow for behavior of the traction drive which is appropriate in terms of vehicle movement dynamics, or the guide system can theoretically even no longer set behavior of the traction drive which is appropriate in terms of vehicle movement dynamics owing to the disconnection of the battery.
As a result of this, the traction drive outputs torques which are undesired or even unacceptable in terms of vehicle movement dynamics, in particular suddenly occurring large negative torques. This then has to be dealt with in the overall concept of such a traction drive of a vehicle by additional measures such as, for example, a mechanical freewheel. These additional measures are costly and extremely undesired since they are required only in the case of a technical fault in the battery and therefore are generally never used.
In the earlier patent application by the applicant with the file number DE 10 2010 041 014 A1, a battery system having a battery which has an output voltage which can be adjusted incrementally was described. The block circuit diagram of a drive system (traction drive) 10 with such a battery 110 is illustrated in FIG. 2. The battery 110 is constructed from a plurality of battery modules 130, 140 which are arranged in an individual battery module line 120 and have battery cells which are connected in series and/or in parallel. The battery modules 130, 140 can be connected or bypassed by means of what are referred to as coupling units in the battery module line 120, which are respectively assigned to the battery modules 130, 140. Systems with such batteries 110 are also referred to as battery direct converters (DICO) 110. They can replace the traction battery 10 illustrated in FIG. 1.
The possible profile of the output voltage UB of the battery direct converter 110 (illustrated in FIG. 2) is illustrated in FIG. 3. The output voltage UB is here the voltage generated by the battery module line 120. FIG. 3 shows that the dependence of the output voltage UB on the number k of the battery modules 130, 140 which are connected to the battery module line 120. The battery modules 130, 140 which are connected to each battery module line 120 each have the same module voltage UM. The output voltage UB of the battery module line 120 which is illustrated as a function of the connected number k of battery modules 130, 140 is linear and follows the relation UB=k·UM, where 1<k<n. Here, n is the maximum number of battery modules 130, 140 which can be connected to the battery module line 120. The maximum output voltage UB of the battery module line 120 can then correspondingly assume the value n·UM.